Such motors comprise a stator and a rotor, the rotor rotating in the stator. The rotor is often, but not necessarily, magnetised. The stator usually comprises salient poles mounting windings forming part of an electrical circuit. By appropriate switching of the circuit the rotor can be driven to rotate at a speed determined by the circuit, usually in a synchronous manner.
There are numerous designs of motor and EP-A-0455578 discloses one arrangement. In this, a hybrid single phase variable reluctance motor is disclosed. Here, two commutating stator poles have two other poles between them, one carrying a permanent magnet to park the rotor in a position in which the motor will start when the commutating poles are energised, and the other carrying a position-sensing device. The arrangement is very simple and effective, with the circuit arrangements being easy to manufacture resulting in a low-cost motor suitable for applications such as driving fans.
However, in this simple mode, the torque produced is very pulsatile, leading to uneven operation.